The invention relates to a sintered boron nitride body and a method for producing same, wherein the boron nitride body is produced for this purpose by at least one pressing process and a subsequent sintering process from a powder made of a hexagonal boron nitride.
Overall, such a boron nitride body made of hexagonal boron nitride (H-BN) has a graphite-like structure and is white in color. In contrast thereto, sintered bodies made of cubic boron nitride (CBN) exhibit greater hardness and are black in color. The latter are used, for example, as cutting materials.
Boron nitride bodies made of hexagonal boron nitride (H-BN) are utilized, for example, for electric insulators, for melting molds for metal melts, for furnace components and as substrates for growing crystals. Due to the use of the hexagonal boron nitride, such bodies have a crystal structure that is formed from layers composed of a planar, hexagonal honeycomb structure. Because of this graphite-like structure, built up from individual planar layers, such boron nitride bodies made of hexagonal boron nitride exhibit strong anisotropy with regard to at least some physical properties, in particular with regard to thermal conductivity or the coefficient of thermal expansion. The properties differ widely depending on the directions in space of perpendicular or parallel to the layers. For instance, thermal conductivity in the direction parallel to the layers, the so-called a-direction, is typically nearly twice as high as in the direction perpendicular thereto. This direction is called the c-direction. This anisotropy can be attributed to the different binding forces between atoms within the individual layers on the one hand and between the layers on the other hand. Depending on the orientation of the body, the properties of such a sintered boron nitride body made of hexagonal boron nitride therefore exhibit strong directional dependence.
For producing such a boron nitride body, the powder is usually pressed to form a cold-pressed molded body, also called a green body, in a first cold-pressing process. The powder used consists to at least virtually 100% of hexagonal boron nitride. Typically, small portions of boron oxide are present, for example in the range of between 1.5 and 2% by weight. The powder usually does not contain any further constituents. This cold-pressed green molded body is a body of low strength. Cold-pressing is carried out without external heat supply, in particular isostatically at a pressing pressure of between 900·105 and 2000·105 Pa. The cold-pressed molded body is subsequently subjected to a second process, namely a hot-pressing process. This is carried out at temperatures of typically 1200 to 1500 degrees, causing the obtained boron oxide to fuse and thus serving as a binder. During hot-pressing, the molded body is maximally compacted to form a hot-pressed molded body. Following hot-pressing, additional sintering is carried out, which is also called a tempering process. Temperatures of about 1800° typically prevail, at which the boron oxide evaporates.
The sintered boron nitride body obtained after the sintering process may subsequently still be mechanically processed in order to obtain the desired final geometric shape. Conventional sintered boron nitride bodies typically have a density of about 1.9 g/cm3.
The high anisotropy is generally considered to be undesirable in such boron nitride bodies. For example, thermal conductivity in a conventional boron nitride body is about 80 W/mK in the c-direction in space and about 130 W/mK in the a-direction in space. Consequently, there is a large difference in thermal conductivity of more than 40 W/mK (at room temperature).
Another problem is the temperature dependence of the thermal conductivity. For instance, in conventional sintered boron nitride bodies, thermal conductivity shows very strong temperature dependence. The thermal conductivity for the c-direction in space, starting from a room temperature to a service temperature in the range of 1000° C., drops to below half and to nearly a third, for example. The strong anisotropy of the thermal conductivity, as well as its strong temperature dependence, therefore pose problems for the user who employs such sintered boron nitride bodies. During temperature changes, for example during heating or also in defined temperature profiles that are run, a person skilled in the art must take a strongly varying thermal conductivity into account. As indicated at the outset, such sintered boron nitride bodies are also used, for example, for molds in metal casting or also in melting furnaces. For such applications, however, knowledge of the thermal conductivity at a given temperature is of crucial importance in order to be able to control the production process, for example a casting process, as effectively as possible.
Finally, the strong anisotropy also poses problems with regard to the coefficient of thermal expansion. Because of the strong anisotropy, it is imperative that care be taken that the boron nitride bodies be used exactly in the defined orientation.